1. Field of the Invention
This invention relates to an operating unit for a key switch incorporated in a key board used as various information inputting means in a point-of-sale (POS) terminal, electronic cash register equipment (ECE) or the like.
1. Description of the Prior Art
There has been so far used widely a key switch in a key board, which has a structure as shown in FIG. 1. The key switch is retained in a mounting hole 2 formed in a metallic support frame 1 by fitting a key switch operating unit 3 into the mounting hole 2 so as to permit the on-off operation of a contact membrane 4. The operating unit 3 is detachably supported on the support frame 1 by means of spring pieces 6 having engaging claws integrally formed on the outside of a housing 5. Within the housing 5, a key stem 7 is guided movably in the vertical direction by a guide cylinder 8 and upwardly urged by a return spring 9. The lower surface of the key stem 7 is opposite to the upper surface of a rubber member 11. Between the key stem 7 and the rubber member 11, a switch pressure spring 10 is interposed. The rubber member 11 is fitted into the lower part of the housing 5 and has a contact pressing protrusion 12 extending downward from at the central portion of the lower surface thereof. A key top 13 is placed on the upper end of the key stem 7.
By pressing down the key top 13, the key stem 7 is moved downward against the return spring 9 so as to push down the contact pressing protrusion 12 of the rubber member 11 through the medium of the switch pressure spring 10, thus depressing the contact membrane 4 to make electrical contact.
Since the conventional key switch operating unit as mentioned above has a clearance gap inevitably formed between the guide cylinder of the housing and the key stem, dust and liquid are permitted to enter into the inside of the key switch in keying, consequently giving rise to increase in sliding resistance, and when the worst comes to the worst, the key switch ceases to function.
Thus, the idea that the housing is closed up tightly to prevent the air from entering into the inside thereof can be considered. However, the key switch in the closed housing having a small internal space entails problem such as difficulty in providing a smooth keying operation. Moreover, the internal pressure in the housing is disadvantageously increased when pressing down the key. Accordingly, there is a possibility that the air increased in pressure may escape out of the housing, thus causing the internal pressure in the housing to be decreased when the key is turned back by the return spring 9. As a result, the key may be possibly prevented from returning in situ.